Inkjet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital data signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand inkjet, individual ink droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. In another process, known as continuous inkjet, a continuous stream of droplets is charged and deflected in an image-wise manner onto the surface of the image-recording element, while un-imaged droplets are caught and returned to an ink sump. Inkjet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
Early inkjet inks were formulated much like conventional printing or pen applied inks. As greater attention has been directed towards printing speed, ease of use, reliability, and environmental issues, and with increasing interest in forming improved images, inks have been formulated to work well on specific media. One challenge is to obtain the highest possible image quality on a variety of inkjet receivers. Typically the receivers are categorized as a photoglossy or plain paper receiver. The two types of receivers are distinguished from one another in that the photoglossy receiver is manufactured with a coated layer above the underlying paper support. Photoglossy receivers may be further categorized as having a swellable polymer coating (non-porous media) or a microporous (hydrophilic particles in binder) media, although hybrid designs are also well known. Typical polymer coated media are capable of very high gloss in excess of 60 gloss units at a viewing angle of 60 degrees. Typical microporous media can be designed to have gloss values approaching those of some polymer coated media. The design of both plain paper and photoglossy media vary widely depending on materials and paper manufacturing processes, which should not be construed to limit the scope of the present invention.
For example, inks intended to provide durable and glossy images on photo-glossy image receivers can incorporate film forming polymers and soluble dye colorants while inks intended to provide well adhering fast drying, smooth images on plain papers can include soluble dye colorants, paper penetrants and paper anti-curl agents. The soluble dye inks all suffer from light fade, a problem which is especially significant when archival photo-images are desired and from poor resistance to rewetting. It has been proposed to alleviate the light fade problem by providing dispersed pigment as colorants in place of soluble dyes. However, use of pigments often leads to a reduction in image gloss and poor rub resistance on photo-glossy media and image inhomogeneity or mottle on plain papers. This inherent mottle or graininess problem which arises when applying pigmented inks to plain papers is generally not an issue when applying dye based inks to the same papers because of the fully soluble nature of the dyes and the more even colorant deposits that arise from them.
Representative disclosures of inkjet inks and printing methods said to provide improved images on glossy or coated papers include: Yatake et al., U.S. Pat. No. 7,030,174, Kurabayashi, U.S. Pat. No. 6,790,878B2, while representative disclosures of inkjet inks and printing methods said to provide improved images on plain papers include: Namba, et al., U.S. Pat. No. 7,094,813, Nakamura et al., U.S. Pat. No. 7,074,843B2, Aritu et al., U.S. Pat. No. 6,695,443B2, Teraoka, et al., U.S. Pat. No. 6,530,656B2, and Kato, U.S. Pat. No. 6,440,203B2.
Disclosures of inkjet inks and printing methods recommended to provide improved images on both glossy papers and economical plain papers include Yatake et al., U.S. Pat. No. 6,890,378B2 which describes the use of specific classes of acetylenic surfactants to provide reduced intercolor bleed properties, and Miyabayashi et al., U.S. Pat. No. 6,864,302 and Tomioka et al., U.S. Pat. No. 6,719,420B2, both of which describe the practice of mixing distinct inks having disparate charge characteristics at the recording material surface. The first suggestion suffers in that it promises to improve only a limited number of the known deficiencies while the second two lead to significant problems in inkjet engine maintenance as the inks can conglomerate in the inkjet apparatus during use.
Oyanagi, U.S. Pat. No. 6,536,891B2 describes control of Yellow to Black (Y v K) intercolor bleed on plain papers imaged by piezo jetting by providing inkjet ink sets with specified relationships between the static surface tensions of light (yellow) and dark (black) high viscosity inks. The high ink viscosity can both limit the jet firing frequency and lead to coalescence when the inks are applied quickly, especially to glossy media. Further no inkjet ink sets having cyan, magenta and yellow inks are disclosed in this publication. Kamoto, et al., US Pub. App. 2004/0069183A1 describes high viscosity inkjet inks with a difference of less than 7 mN/m between the static and dynamic surface tension at surface refresh rates of 0.5 to 35 Hz (i.e. surface ages of 2 s to ˜30 ms) relative to the static surface tension of the same ink as providing inks with desired discharge stability from a piezo inkjet ink application system and a high quality recorded image. Again, high ink viscosity can both limit the jet firing frequency and lead to coalescence when the inks are applied quickly, especially to glossy media. Further, no explanation is given in this publication of bow to derive the single quoted value for dynamic surface tension over the stated surface refresh ages from the range of distinct values which inherently follow from use of a dynamic surface tension agent in an ink and are inherently reported at these distinct ink surface ages when using a bubble tensiometer.
Homna, et al., U.S. Pat. No. 7,037,362B2 describes dye based colored inks and pigment based black inks, characterized in having only limited viscosity after evaporation and a specified relationship between the dynamic surface tension of the same ink at surface ages of 10 ms and 1 s, that are said to provide quick drying and limited image bleeding on plain papers. Here, the use of dye based inks inherently leads to image instability on storage. Further, the utility of the approach is limited to dye based inks since pigment based inks set-up under the test conditions and inherently exhibit very high dry down viscosities. Further, no color inkjet ink sets with cyan, magenta and colorant inks are disclosed in this publication.
Koga, US Pub. App. 2007/0022902A1 describes high viscosity high dynamic surface tension dye based inks having reduced intercolor bleed on plain papers, and characterized by exhibiting a specified relationship within the ink set between the variously colored ink dynamic surface tensions at surface ages of 30 ms and 1 s. Here, the use of dye based inks inherently leads to image instability on storage. Further, since this publication discussed only dye colorant inks, it provides no teaching relative to the use of pigment colorant inks with respect to plain paper graininess. Ma, et al., US Pub App. 2007/0120928A1 describes inkjet inks having reduced intercolor bleed on glossy photo papers, and characterized by exhibiting a specified relationship within the ink set between the variously colored ink dynamic surface tensions. Specific ink components, formulations and ink physical properties beyond surfactant identity and levels are not disclosed. Further only black and yellow inks are described and no color inkjet ink sets having cyan, magenta and yellow inks are disclosed. Sekiguchi, US Pub. App. 2007/0139501A1 describes yellow and black dye based inkjet inks, suitable for use in thermal inkjet printers, the inks said to exhibit high density and reduced intercolor bleed (Yellow v Black) on plain papers when formulated based on the dynamic surface tension relationships of related model inks with no colorant added. The relationship between the actual ink properties and their performance is not disclosed. Here again, the use of dye based inks inherently leads to image instability on storage. Further only black and yellow inks are described and no color inkjet ink sets having cyan, magenta and yellow inks are disclosed.
Thus, none of this art provides inkjet inks, inkjet ink sets or inkjet printing methods which can provide high gloss archival images on micro-porous photo-glossy or coated papers, while simultaneously providing low noise images on economical plain papers. This has led to a situation where end users remain faced with the need to choose either between inkjet printer systems designed to produce photo-glossy images or inkjet printer systems designed for plain paper output. Thus, there is an unmet need for inkjet inks, inkjet ink sets, and inkjet printing systems which can provide excellent images when used with both photo-glossy coated papers and economical plain papers.
Ongoing theoretical and practical studies of the dynamics of the movement of water or oils through soils and the impregnation of woods and other porous materials with liquids led toward Washburn's seminal theoretical description of the physics and dynamics of capillary flow at Washburn, E. W. The Physical Review, Vol. XVII, No. 3, pages 273-283 (1921). Here, Washburn disclosed that the rate of fluid flow into a capillary was proportional to the fluid surface tension and inversely proportional to the fluid viscosity under conditions where the fluid wets the capillary surface. Washburn draws a parallel between the rate of single capillary flow and the overall rate of fluid uptake that might be encountered during the wetting of porous bodies. The import of this work is intuitively in that increasing viscosity tends to retard the flow of fluid into porous material while decreasing fluid surface tension retards flow and mixing of fluids with hydrophilic surfaces, such as the aforesaid mentioned soil and wood. This work considers equilibrium processes and bulk flows over long times and does not consider fluids whose surface tension might vary with the age of the fluid surface.
Typically, molecularly homogeneous fluids exhibit surface tensions that do not vary with the age of the fluid surface. It is only with molecularly heterogeneous bulk fluids, i.e. mixed ingredient solutions, that changes in surface tension with surface age occur and only when these heterogeneities involve the segregation of particular molecular species between the fluid bulk and the fluid surface. A most relevant occurrence of this phenomenon occurs with fluids that are aqueous solutions of water soluble organic materials where the water soluble organic materials are distinguished by having rod-like structures with hydrophobic and hydrophilic ends. At equilibrium, hydrogen bonding forces between water molecules tend to exclude the hydrophobic ends of the rod-like structures from the bulk with the result that the overall fluid is heterogeneous with the organic materials concentrated at the fluid surface and with hydrophobic ends of the organic materials aligned away from the bulk. As a result the fluid exhibits a lowered surface tension more characteristic of organic fluids than aqueous fluids. It is precisely because of these surface altering properties that rod-like organic material with hydrophilic and hydrophobic ends are referred to as surfactants.
When a bulk fluid, such as an inkjet ink, having an incorporated surfactant is fired by an ink-ejector as an ink droplet, the droplet initially can be in a non-equilibrium condition as far as distribution of the surfactant between bulk and surface are concerned. The distinction between bulk and surface properties only being re-established as surfactant molecules accumulate at the freshly formed ink-drop surface under the impulse of random molecular migration. This random molecular migration, often quantified as a molecular diffusion coefficient, is in turn influenced inter alia by the size of the diffusing molecule, which usually follows the molecular weight, and by the viscosity of the bulk fluid. As any particular ink drop impacts a recording medium, the surface and bulk of the drop are mechanically mixed and the equilibrium properties of the ink become less relevant to its physical interaction with the medium.